
For a picture of the full and official (as far as we know) roster, CLICK HERE (spoilers inside). The below contains major spoilers of the game (characters/stages), so read ahead only if you dare.
New Characters:
Lucario (Pokemon)
R.O.B. (Gyromite)
Star Wolf (Starfox)
Toon Link (Zelda)
Returning Characters:
Captain Falcon
Falco
Ganondorf
Jigglypuff
Luigi
Marth
Mr. Game & Watch
Ness
New Stages:
Something that looks like Final Destination
Classic Mario Bros. (Mario Bros.)
Green Hill Zone (Sonic the Hedgehog)
Pirate Ship (TLoZ: Wind Waker)
Port Town (F-Zero)
Luigi's Mansion (Luigi's Mansion)
Electroplankton (Electroplankton)
Also:
Ridley is a boss in Subspace Emissary, though he is probably not playable.
That's 35 confirmed characters (not counting Sheik and Zero Suit Samus), and 28 confirmed stages (not counting Melee stages)
All of the extraneous details, along with quite a few screenshots, can be found on Joystiq.
EDIT 1: Forgot to list one of the stages.
[via Joystiq]